Rear defoggers located on a vehicle backglass or rear windshield may include a series of resistive conductors located in or on the rear windshield. When electrical power is applied, the resistive conductors are activated or heated to evaporate condensation from the rear windshield. Generally during a drive cycle, the resistive conductors are supplied electrical power for a predetermined amount of time (e.g., generally between about 10-20 minutes), and electrical power is then turned off to conserve electrical energy and improve fuel economy.
In some types of vehicles, that rear windshield tends to fog up relatively quickly once the electrical power is turned off. Thus, a driver may need to turn on or activate the rear defogger relatively often, which in turn may cause annoyance or irritation. In particular, the rear windshield tends to fog up more quickly if the number of occupants in the vehicle increases. In one approach, a humidity sensor may be provided to determine the humidity within the interior cabin of the vehicle. The electrical power provided to the resistive conductors may be activated based on the humidity reading from the humidity sensor. However, the humidity sensor may also generate inaccurate readings during the early stages of the drive cycle. Accordingly, it is desirable to provide a cost-effective and efficient system that cycles the rear defogger on and off during a drive cycle.